Special declaration votes are made by registered electors who are unable to vote at a polling place in their own electorate on election day, or whose names do not appear on the printed electoral roll.
Who can cast a special declaration vote?

You can make a "provisional enrolment" as soon as you turn 17 by making a standard enrolment application described under How to enrol to vote. On your 18th birthday you will be put on the electoral roll automatically.

Enrolling to vote is easy. You will not be able to vote if you are not enrolled before election day. It is compulsory to be enrolled if you are qualified, although voting is optional.
Can I enrol online?